In a solid polymer fuel cell, a laminated body, in which, on both sides of a planar MEA (Membrane Electrode Assembly), a separator is laminated, is regarded as one unit, and plural units are stacked and form a fuel cell stack. The MEA is formed as a three layer structure in which, between a pair of gas diffusion electrodes that constitute a cathode and an anode, an electrolyte membrane made of, for example, an ion exchange resin, is interposed. In the gas diffusion electrode, outside of an electrode catalyst layer in contact with an electrolyte membrane, a gas diffusion layer is formed. Furthermore, the separator, laminated so as to be in contact with the gas diffusion electrode of the MEA, is provided with a gas passage that allows a gas to flow and a coolant passage between the separator and the gas diffusion electrode. According to such a fuel cell, for example when a hydrogen gas as a fuel is allowed to flow in the gas passage facing the gas diffusion electrode on the anode side, and an oxidizing gas such as oxygen or air is allowed to flow in the gas passage facing the gas diffusion electrode on the cathode side, there occurs an electrochemical reaction, resulting in the generation of electricity.
The separator must function so that, while electrons generated at the anode side according to a catalytic reaction of the hydrogen gas are supplied to an external circuit, electrons from the external circuit may be supplied to the cathode side. Accordingly, for the separator, a conductive material made of a graphite-based material or a metal-based material is used, and in particular the metal based material is regarded as being advantageous in view of superiority in mechanical strength and in ability to be made lighter and more compact by being formed into a thin plate. As a metallic separator, one in which, for example, a thin plate of stainless steel on a surface of which conductive inclusions that form a conductive passage are dispersed and exposed, is press-formed into the gas passage and the coolant passage can be cited. However, these separators have subjects (1) to (5) shown as follows. Hereinafter, these subjects and purposes of the present invention corresponding to each subject will be explained.
(1) As a metallic separator, one in which stainless steel is press-formed, in which nonmetal conductive inclusions that form a conductive passage are dispersed in the metallic structure, can be cited. This separator appears to be promising economically. However, in the stainless steel plate, the entire area of the matrix surface has a natural oxide film. Therefore, contact resistance against the MEA is high in this case, whereby characteristics for generation of electrical energy are decreased. Accordingly, after the press-forming process, a process in which the conductive inclusions made to project at the surface is performed, resulting in decreasing the contact resistance. As a process in which the conductive inclusions are made to project, for example, a means in which a matrix on the surface is eliminated by an electrolytic etching may be adopted.
However, when the conductive inclusions are merely projected from the matrix surface, a portion of the conductive inclusions can fall off from the surface, whereby a decrease of contact resistance cannot be greatly obtained.
Accordingly, the present invention first intends to provide a metallic separator for a fuel cell, in which the falling off of conductive inclusions projecting from a matrix surface is prevented, whereby the contact resistance would be decreased, resulting in increasing the characteristics for generation of electrical energy, and a production method for the same.
(2) The said gas passage and the said coolant passage are composed of grooves having two faces obtained by bending a material plate into a cross-section shape having concavities and convexities. In an outside of an R-portion which is made into a corner by the bending process, the amount of elongation on the surface is increased by tensile stress. Therefore, cracks are easily formed at boundary faces between the matrix and the conductive inclusions. FIG. 1 is a pattern diagram of this phenomenon. At the outside of the R-portion of separator 10A obtained by bending material plate 10, cracks are formed at the boundary face between matrix 20 and conductive inclusions 30. This generation of cracks induces fall off of the conductive inclusions and gap corrosion beginning at the cracks, whereby a function as a separator is decreased.
Accordingly, the present invention secondly intends to provide a metallic separator for a fuel cell, in which falling off of the conductive inclusions and forming of the cracks in boundaries between the conductive inclusions and the matrix by the bending process are suppressed, resulting in ensuring the function as a separator, and a production method for the same.
(3) The said gas passage and the said coolant passage are composed of grooves having two faces obtained by bending a material plate into a cross-section shape having concavities and convexities. In the outside of the R-portion which is the corner by the bending process, the amount of an elongation on the surface is increased by tensile stress. Therefore, cracks can be formed at the boundary face between the matrix and the conductive inclusions, whereby the conductive inclusions can fall off. In a case of using a separator in which the conductive inclusions fall off, pitting corrosion beginning at the falling off mark are generated during operation of the fuel cell, whereby the corrosion is progressed.
Accordingly, the present invention thirdly intends to provide a production method for a metallic separator for a fuel cell, in which falling off of the conductive inclusions in the press-forming is suppressed, resulting in the production of a robust separator.
(4) In the said case of using a separator in which the conductive inclusions fall off, pitting corrosion beginning at the falling off mark is generated during operation of the fuel cell, whereby the corrosion is proceeded as mentioned above, and moreover, the contact resistance is increased. Furthermore, the cracks beginning at the conductive inclusions are easily formed, depending on the shape of the conductive inclusions, and the forming of the cracks may be promoted. FIG. 2 shows an occurrence of a crack beginning at conductive inclusions 60 by a bending process in producing a separator 40A by press-forming material plate 40 of the separator. As shown in FIG. 2, in particular, conductive inclusion 60, which is precipitated in the center of the material plate 40 and in which the lengthwise length is longer than the crosswise length in cross-section, acts as a wedge hammered against matrix 50, whereby the crack 50a is promoted. In such a case, leak of fuel gas and oxidizing gas occurs, resulting in decreasing function as a separator.
Accordingly, the present invention fourthly intends to provide a production method for a metallic separator for a fuel cell, in which falling off of the conductive inclusions in the press-forming and the forming of the cracks beginning at conductive inclusions are prevented, resulting in the ability to produce durable.
(5) As a process in which the conductive inclusions are projected, the said means for the matrix on the surface to be eliminated by etching can be cited. In the case of performing the process in which the conductive inclusions project, a portion of the conductive inclusions can fall off from the surface, whereby a decreasing effect of a contact resistance was not largely obtained. For example, in the case of causing the conductive inclusions to project at the surface by an electrolytic etching, the matrix is dissolved in the process. When this happens, a portion of the matrix in the vicinity of boundary face between the matrix and the conductive inclusions is particularly easy to be selectively dissolved, whereby pitting corrosion is generated around the conductive inclusions, resulting in it being easy for the conductive inclusions to fall off.
Accordingly, the present invention fifthly intends to provide a production method for a metallic separator for a fuel cell, in which generation of pitting corrosion in the boundary face between the matrix and the conductive inclusions is prevented, in the process of projecting the conductive inclusions on the surface, whereby falling off of the conductive inclusions is prevented, resulting in decreasing the contact resistance, and accordingly increasing the characteristics for generation of electrical energy.